Semiconductor devices and liquid crystal display devices are fabricated by a so-called photolithography technique, wherein a pattern formed on a mask is transferred onto a photosensitive substrate. An exposure apparatus used in this photolithographic process comprises a mask stage that supports a mask as well as a substrate stage that supports a substrate, and transfers the pattern of the mask onto the substrate through a projection optical system while successively moving the mask stage and the substrate stage. In recent years, there has been demand for higher resolution projection optical systems in order to handle the much higher levels of integration of device patterns. The shorter the exposure wavelength used and the larger the numerical aperture of the projection optical system, the higher the resolution of the projection optical system. Consequently, the exposure wavelength used in exposure apparatuses has shortened year by year, and the numerical aperture of projection optical systems has increased. Furthermore, the current mainstream exposure wavelength is 248 nm, which is the wavelength of light produced by a KrF excimer laser, but an even shorter 193 nm wavelength ArF excimer laser is also being commercialized.
In addition, as with resolution, the depth of focus (DOF) is important when performing an exposure. The following equations express the resolution R and the depth of focus δ, respectively.R=k1·λ/NA  (1)δ=±k2·λ/NA2  (2)
Therein, λ is the exposure wavelength, NA is the numerical aperture of the projection optical system, and k1 and k2 are the process coefficients. Equations (1) and (2) teach that if the exposure wavelength λ is shortened and the numerical aperture NA is increased in order to enhance the resolution R, then the depth of focus δ decreases.
If the depth of focus δ becomes excessively small, then it will become difficult to align the front surface of the substrate with the image plane of the projection optical system, which creates a risk that the margin of focus will be insufficient during the exposure operation. Accordingly, a liquid immersion method has been proposed, as disclosed in, for example, Patent Document 1 below, as a method to substantially shorten the exposure wavelength and increase the depth of focus. This liquid immersion method forms an immersion area by filling a liquid, such as water or an organic solvent, between the lower surface of the projection optical system and the front surface of the substrate, thus taking advantage of the fact that the wavelength of the exposure light in a liquid is 1/n that of in air (where n is the refractive index of the liquid, normally about 1.2-1.6), thereby improving the resolution as well as increasing the depth of focus by approximately n times.
PATENT DOCUMENT 1: PCT International Publication WO99/49504